Apparatus for and method of predictive time stamping of isochronous data packets transmitted over an IEEE 1394-1995 serial bus network

ABSTRACT

In order to predictively time stamp isochronous data packets transmitted over an IEEE 1394-1995 serial bus network, an application, which is to send a stream of isochronous data packets to a receiving node, first transmits a number of dummy frames each consisting of a number of packets. Preferably, these isochronous data packets make up frames of video data. From these dummy packets, the application obtains the time stamp values within the common isochronous packet (CIP) header of each packet. Using these obtained time stamp values, the application calculates a presentation time value for each data frame to be transmitted. The obtained time stamp value from a transmitted video frame is used to calculate the presentation time for a video frame which is a number of frames ahead within the transmit queue. Once the presentation time value for a frame is calculated, that value is then inserted by the application into the CIP header for the first packet within that frame and the frame is sent to the transmit queue for transmission to the receiving node over the IEEE 1394-1995 serial bus network.

FIELD OF THE INVENTION

The present invention relates to the field of transmitting informationbetween devices. More particularly, the present invention relates to thefield of transmitting time sensitive information between devices over anIEEE 1394-1995 serial bus network.

BACKGROUND OF THE INVENTION

The IEEE 1394-1995 standard, “1394-1995 Standard For A High PerformanceSerial Bus,” is an international standard for implementing aninexpensive high-speed serial bus architecture which supports bothasynchronous and isochronous format data transfers. Isochronous datatransfers are real-time transfers which take place such that the timeintervals between significant instances have the same duration at boththe transmitting and receiving applications. Each packet of datatransferred isochronously is transferred in its own time period. Anexample of an ideal application for the transfer of data isochronouslywould be from a video recorder to a television set. The video recorderrecords images and sounds and saves the data in discrete chunks orpackets. The video recorder then transfers each packet, representing theimage and sound recorded over a limited time period, during that timeperiod, for display by the television set. The IEEE 1394-1995 standardbus architecture provides multiple channels for isochronous datatransfer between applications. A six bit channel number is broadcastwith the data to ensure reception by the appropriate application. Thisallows multiple applications to concurrently transmit isochronous dataacross the bus structure. Asynchronous transfers are traditional datatransfer operations which take place as soon as possible and transfer anamount of data from a source to a destination.

The IEEE 1394-1995 standard provides a high-speed serial bus forinterconnecting digital devices thereby providing a universal I/Oconnection. The IEEE 1394-1995 standard defines a digital interface forthe applications thereby eliminating the need for an application toconvert digital data to analog data before it is transmitted across thebus. Correspondingly, a receiving application will receive digital datafrom the bus, not analog data, and will therefore not be required toconvert analog data to digital data. The cable required by the IEEE1394-1995 standard is very thin in size compared to other bulkier cablesused to connect such devices. Devices can be added and removed from anIEEE 1394-1995 bus while the bus is active. If a device is so added orremoved the bus will then automatically reconfigure itself fortransmitting data between the then existing nodes. A node is considereda logical entity with a unique address on the bus structure. Each nodeprovides an identification ROM, a standardized set of control registersand its own address space.

The IEEE 1394-1995 cable environment is a network of nodes connected bypoint-to-point links, including a port on each node's physicalconnection and the cable between them. The physical topology for thecable environment of an IEEE 1394-1995 serial bus is a non-cyclicnetwork of multiple ports, with finite branches. The primary restrictionon the cable environment is that nodes must be connected togetherwithout forming any closed loops.

The IEEE 1394-1995 cables connect ports together on different nodes.Each port includes terminators, transceivers and simple logic. A nodecan have multiple ports at its physical connection. The cable and portsact as bus repeaters between the nodes to simulate a single logical bus.The cable physical connection at each node includes one or more ports,arbitration logic, a resynchronizer and an encoder. Each of the portsprovide the cable media interface into which the cable connector isconnected. The arbitration logic provides access to the bus for thenode. The resynchronizer takes received data-strobe encoded data bitsand generates data bits synchronized to a local clock for use by theapplications within the node. The encoder takes either data beingtransmitted by the node or data received by the resynchronizer, which isaddressed to another node, and encodes it in data-strobe format fortransmission across the IEEE 1394-1995 serial bus. Using thesecomponents, the cable physical connection translates the physicalpoint-to-point topology of the cable environment into a virtualbroadcast bus, which is expected by higher layers of the system. This isaccomplished by taking all data received on one port of the physicalconnection, resynchronizing the data to a local clock and repeating thedata out of all of the other ports from the physical connection.

When transmitting isochronous data between two devices, each packet ofisochronous data is time-stamped with the current bus time of the cyclein which the packet is transmitted. If not received by the receivingdevice in the correct cycle, the packet is typically discarded by thereceiving device and the data is lost. This is especially true whentransmitting video data which is very time sensitive. When transmittingframes of video data, the first packet of the received frames of datahave to be received within a recognized boundary of time as compared tothe time stamp value of the packet. If the first packet of the frame isreceived outside of this boundary, the entire frame is generallydiscarded and not processed by the receiving device.

The value of this time stamp is acquired from the cycle time register,within the transmitting device, which maintains the current bus time fora node. The cycle time register includes a second_count field and acycle_count field which together form a value representing the currentcycle. This cycle value is incremented on each carry from a cycle_offsetfield. The cycle_offset field is updated on each transition of thesystem clock. On the transition after the value within the cycle_offsetfield is equal to 3071, the value within this field wraps around to zeroand the value within the cycle_count field is incremented. The valuewithin the cycle offset field is a fractional part of the currentisochronous cycle. When transmitting data from an application within thenode, the application must obtain the current bus time from the cycletime register, then load the current bus time value into the packet andtransmit the packet over the IEEE 1394-1995 serial bus to the receivingnode. Within the transmitting node, there can be a substantial delaybetween the time in which the current bus time value is sent from thecycle time register and the time at which the application receives thecurrent bus time value from the cycle time register, inserts it into thepacket and actually transmits the packet over the IEEE 1394-1995 serialbus network. If this delay is significant, the current bus time valuereceived by the application may be obsolete and outside of theappropriate boundary of time, by the time the packet is actuallytransmitted, causing the transmitted packets to be discarded by thereceiving device.

What is needed is a method of and apparatus for ensuring thattransmitted packets will be received by the receiving device within theappropriate boundary of time in-order that the packets are properlyprocessed by the receiving device and not discarded. What is furtherneeded is a method of and apparatus for predicting the current bus timevalue corresponding to the actual transmission of isochronous packetsfrom a node on an IEEE 1394-1995 serial bus.

SUMMARY OF THE INVENTION

In order to predictively time stamp isochronous data packets transmittedover an IEEE 1394-1995 serial bus network, an application, which is tosend a stream of isochronous data packets to a receiving node, firsttransmits a number of dummy frames each consisting of a number ofpackets. Preferably, these isochronous data packets make up frames ofvideo data. From these dummy packets, the application obtains the timestamp values within the common isochronous packet (CIP) header of eachpacket. Using these obtained time stamp values, the applicationcalculates a presentation time value for each data frame to betransmitted. The obtained time stamp value from a transmitted videoframe is used to calculate the presentation time for a video frame whichis a number of frames ahead within the transmit queue. Once thepresentation time value for a frame is calculated, that value is theninserted by the application into the CIP header for the first packetwithin that frame and the frame is sent to the transmit queue fortransmission to the receiving node over the IEEE 1394-1995 serial busnetwork.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an IEEE 1394-1995 serial busnetwork including a computer system and a video camera.

FIG. 2 illustrates a block diagram of the internal components of thecomputer system 10.

FIG. 3 illustrates a format of an isochronous data packet fortransmission over an IEEE 1394-1995 serial bus network.

FIG. 4 illustrates a format of a CIP header field within an isochronousdata packet.

FIG. 5 illustrates a flow diagram of the steps involved in predictivelytime stamping isochronous data frames according to the preferredembodiment of the present invention.

FIG. 6 illustrates the transmission of a stream of isochronous videoframes including a number of dummy frames preceding the actual videoframes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A block diagram of an exemplary IEEE 1394-1995 serial bus networkincluding a computer system and a video camera is illustrated in FIG. 1.The computer system 10 includes an associated display 12 and is coupledto the video camera 14 by the IEEE 1394-1995 serial bus cable 16. Videodata and associated data are sent between the video camera 14 and thecomputer 10 over the IEEE 1394-1995 serial bus cable 16.

A block diagram of the internal components of the computer system 14 isillustrated in FIG. 2. The computer system 10 includes a centralprocessor unit (CPU) 20, a main memory 30, a video memory 22, a massstorage device 32 and an IEEE 1394-1995 interface circuit 28, allcoupled together by a conventional bidirectional system bus 34. Theinterface circuit 28 includes the physical interface circuit 42 forsending and receiving communications on the IEEE 1394-1995 serial bus.The physical interface circuit 42 is coupled to the camera 14 over theIEEE 1394-1995 serial bus cable 16. In the preferred embodiment of thepresent invention, the interface circuit 28 is implemented on an IEEE1394-1995 interface card within the computer system 10. However, itshould be apparent to those skilled in the art that the interfacecircuit 28 can be implemented within the computer system 10 in any otherappropriate manner, including building the interface circuit onto themotherboard itself. The mass storage device 32 may include both fixedand removable media using any one or more of magnetic, optical ormagneto-optical storage technology or any other available mass storagetechnology. The system bus 34 contains an address bus for addressing anyportion of the memory 22 and 30. The system bus 34 also includes a databus for transferring data between and among the CPU 20, the main memory30, the video memory 22, the mass-storage device 32 and the interfacecircuit 28.

The computer system 10 is also coupled to a number of peripheral inputand output devices including the keyboard 38, the mouse 40 and theassociated display 12. The keyboard 38 is coupled to the CPU 20 forallowing a user to input data and control commands into the computersystem 10. A conventional mouse 40 is coupled to the keyboard 38 formanipulating graphic images on the display 12 as a cursor controldevice.

A port of the video memory 22 is coupled to a video multiplex andshifter circuit 24, which in turn is coupled to a video amplifier 26.The video amplifier 26 drives the display 12. The video multiplex andshifter circuitry 24 and the video amplifier 26 convert pixel datastored in the video memory 22 to raster signals suitable for use by thedisplay 12.

A format of an isochronous data packet for transmission over an IEEE1394-1995 serial bus network is illustrated in FIG. 3. The format of thedata packet also complies with the IEC 1883 standard. The data_lengthfield contains a value representing the number of bytes of data withinthe data field, including the number of bytes within the CIP header. Thechannel field contains the channel number on which the isochronouspacket is transmitted. The tCode field contains the transaction code forthe packet. For isochronous data packets, the tCode field containseither a value of Ah or Ch. The sy field contains a synchronization flagused in some applications to synchronize the data in the currentisochronous packet with some application specific event. The sourceIDfield contains a six bit value representing the physical identifyingcode of the node which is transmitting the packet. The values in theother CIP header fields depend on the format of the data beingtransmitted in the packet. The data field, if present, contains thecontent data being transmitted in the packet.

A format of the CIP header within an isochronous data packet isillustrated in FIG. 4. Within the CIP header, the SID field contains thesource node ID value of the transmitting node. The DBS field contains avalue representing the size of the data block in quadlets. The FN fieldcontains a fraction number representing the number of data blocks intowhich a source packet is divided. The QPC field contains a valuerepresenting the number of dummy quadlets added to a source packet toequalize the size of the divided data blocks. If the FN field indicatesthat the source packet is not divided, then the QPC field will contain avalue equal to zero. The SPH flag represents whether or not the sourcepacket includes a source packet header. The SPH flag is set equal to alogical “one” when the source packet does include a source packetheader. The rsv field is reserved for future extension. The DBC field isthe continuity counter of data blocks to detect a loss of data blocks.The FMT field includes a format identifier which identifies the formatof the packet. The FDF field is a format dependent field and depends onthe format of the packet. The SYT field is used to synchronize thetransmitter and the receiver.

When transmitting isochronous data over an IEEE 1394-1995 serial busnetwork, the SYT field includes a time stamp value for the presentationtime of the frame. The receiving node uses this time stamp value toensure that the data is presented within the correct boundary of timefor video data. As discussed above, if the frame does not fall withinthe correct boundary of time, the receiving device will discard theentire frame.

In order to ensure that the transmitted data includes the appropriatetime stamp value, the application of the present invention calculatesthe time stamp value by first sending a number of dummy data frames. Adata frame consists of a number of isochronous packets. The number ofisochronous packets per frame is dependent upon the type of data beingsent. In terms of video, a data frame can be a single frame of digitalvideo. After the first data frame is sent, a time stamp of the lastpacket sent of the first frame is obtained. From this value, a new timestamp value or presentation time is calculated for the first packet of avideo frame which is a number of frames ahead of the frame from whichthe time stamp value was obtained. In the preferred embodiment of thepresent invention, the presentation time is calculated for a video framewhich is four frames, or one less than the preferred number of dummyframes, ahead of the frame from which the time stamp value was obtained.Once calculated, this presentation time value is written into the SYTfield of the CIP header of the first packet within the appropriate dataframe and sent to the transmit queue for transmission over the IEEE1394-1995 serial bus to the receiving node.

A flow diagram of the steps involved in predictively time stampingisochronous data frames according to the preferred embodiment of thepresent invention, is illustrated in FIG. 5. The flow chart is enteredat the step 50, when an application within the computer 10 is preparingto transmit a stream of isochronous data to the video camera 14. At thestep 52, the application attaches the dummy data frames to theisochronous transmit queue. Each dummy data frame consists of a numberof dummy IEEE 1394-1995 isochronous packets. The number of dummy packetsper data frame is determined by the type of data being sent. At the step54, the application starts to transmit the dummy frames from thecomputer 10, over the IEEE 1394-1995 serial bus network 16 to the videocamera 14. In the preferred embodiment of the present invention fivedummy data frames are sent before actual data frames containing actualvideo data are transmitted to the receiving device. Alternatively, anyappropriate number of dummy data frames can be transmitted by theapplication. At the step 56, the application waits until it receivesnotification that a dummy data frame has been sent.

After the first data frame is sent, the application obtains the timestamp value of the last packet in this data frame from the IEEE1394-1995 interface circuit. From this time stamp value, thepresentation time for the first real packet within the first actual dataframe is calculated at the step 58. The first actual data frame will bethe sixth frame to be sent and will be transmitted after the five dummydata frames. This presentation time value is calculated using thefollowing equation:PresentationTime = ((#  of  Dummy  Frames − 1) * DataFrameXmtTimeLength) + TimeStamp + PresentationTimeFactorThe DataFrameXmtTimeLength value represents the time length of each dataframe that is transmitted and is calculated by multiplying the number ofisochronous packets per data frame by the isochronous transmission timeper packet. The isochronous transmission time per packet is a fixedvalue per packet as described in the IEEE 1394-1995 specification. TheDataFrameXmtTimeLength value is multiplied by a value equal to one lessthan the number of dummy data frames being sent, in order to add anappropriate time value representing the number of transmitted framesbetween the frame from which the time stamp value was obtained and theframe for which the presentation time is being calculated. The TimeStampvalue is the time stamp value retrieved from the transmission of aprevious packet within a data frame. For the first actual video frame,the TimeStamp value is equal to the cycle time of the last isochronouspacket sent in the first dummy frame. Within the preferred embodiment ofthe present invention, only the cycle count value from the cycle time ofthe last isochronous packet sent in the first dummy frame is used as theTimeStamp value. The cycle offset value within the cycle time of thelast isochronous packet sent in the first dummy frame is not usedbecause only the cycle that the packet is sent is needed to calculatethe presentation time of the packet. The offset time within the cycle isnot necessary to calculate the presentation time. It should however beapparent to those skilled in the art that the value of the entire cycletime could alternately be used as the TimeStamp value. ThePresentationTimeFactor is a factor in units of isochronous cycles whichis used to change the time stamp value into an appropriate presentationtime. The value of the PresentationTimeFactor is dependent on thespecific implementation and will vary due to system implementation andperformance characteristics. The PresentationTimeFactor value iscomposed of two components and is calculated using the followingequation:PresentationTimeFactor=tdiff+TransmissionDelayLimitThe tdiff value represents the difference between the actual cycle timewhen a packet is transmitted and the cycle time that the hardwareactually reports that the packet has been transmitted. The delay betweenthe actual cycle time when a packet is transmitted and the cycle timethat the hardware actually reports that the packet has been transmittedis determined by the specific hardware implementation. If the hardwaresupports returning the actual time that the packet was transmitted, thenthe tdiff value is equal to zero. The TransmissionDelayLimit value isequal to the maximum number of cycles allowed for a packet to betransmitted by the audio/video device. This value is described in the“Specifications of Consumer-Use Digital VCRs” for specific hardwaredevices. Preferably, this value is equal to 450 microseconds or threecycles.

Once the presentation time is calculated for a data frame, this dataframe can be time stamped. At the step 60, the presentation time valueis written into the SYT field of the first isochronous data packetcontained within the data frame. At the step 62 the data frame is thenattached to the transmit queue for transmission over the IEEE 1394-1995serial bus network. It is then determined, at the step 64, if there aremore data frames within this stream of data to be transmitted. If thereare more data frames to be transmitted, steps 56 through 62 are repeatedfor each frame to be transmitted in order to calculate an appropriatepresentation time value for each frame from an obtained time stamp valuefrom the transmitted frame which is four frames before the frame forwhich the presentation time value is being calculated. Once all framesare appropriately time stamped and attached to the transmit queue, theoperation is finished at the step 66.

The transmission of a stream of isochronous frames including a number ofdummy frames preceding the actual video frames is illustrated in FIG. 6.The five dummy frames 80, 82, 84, 86 and 88 are transmitted successivelybefore the actual video frames 90 and 92. From the five dummy frames 80,82, 84, 86 and 88, the application calculates the presentation timevalues for the actual video frames within the data stream. As describedin detail above, the presentation time value for the actual frame 90 iscalculated using the time stamp value obtained from the first dummyframe 80. Correspondingly, the presentation time value for the secondactual frame 92 is calculated using the time stamp value obtained fromthe second dummy frame 82. Once the presentation time value for a frameis calculated, this presentation time value is written into the SYTfield of the CIP header of the first packet within the frame. Theremaining packets within the frame are time-stamped in a conventionalmanner with the current bus time of the cycle in which the frame istransmitted. Once the presentation time value for a frame is calculatedand written into the SYT field of the CIP header, the frame is thenadded to the transmitting node's transmit queue for transmission overthe IEEE 1394-1995 serial bus to the receiving node at the appropriatetime.

This process is repeated for each data frame in the stream of data untilall of the data frames within the stream of data have been transmittedfrom the computer 10 to the video camera 14. In this manner, bycalculating the appropriate time stamp value for a frame and insertingthat value into the SYT field of the CIP header of the first packetwithin the frame, the application ensures that when received by thevideo camera 14, the frames of video data will be properly processed andnot discarded.

The preferred embodiment of the present invention is used to transmitisochronous data over an IEEE 1394-1995 serial bus network from asoftware application within a personal computer 10 to a video camera 14.However, it should be apparent to those skilled in the art that thepresent invention can be used to transmit time sensitive data frames andpackets between any two appropriately configured applications and/ordevices in order to ensure that when received, the time stamp value isappropriate for the cycle in which the frame or packet of data istransmitted. Within the preferred embodiment of the present invention,the time stamp value for an entire frame is calculated and written intothe first packet of the frame. It should be apparent to those skilled inthe art that the present invention can also be used to appropriatelytime stamp multiple individual packets whether or not they are part of avideo frame.

The present invention has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the invention. Suchreference herein to specific embodiments and details thereof is notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications may be made inthe embodiment chosen for illustration without departing from the spiritand scope of the invention.

1-30. (canceled)
 31. A method of transmitting data over a network,comprising: a. transmitting a frame of data from a first node to asecond node; b. timestamping the frame of data with a time stamp valueat the second node; c. determining if the time stamp value is within acorrect boundary time for video data; d. processing the frame of datawithin the correct boundary of time for video data; and e. discardingthe frame of data not within the correct boundary of time for videodata.
 32. An application for transmitting data from a computer to avideo camera over a serial bus network.
 33. An application for obtaininga time stamp value from a previous packet, calculating a time value fora packet to be transmitted a number of packets after the previous packetusing the obtained time stamp value and inserting the time value intothe packet to be transmitted before the packet to be transmitted istransmitted.